The present invention relates to metal casting and can be used in producing large sized blades with directional and single crystal structure having large horizontal shoulders.
Apparatuses for casting blades by directional solidification methods are known. Such apparatuses include a heating zone and a cooling zone, which are separated by a horizontal shield or baffle. In instances where a chill plate acts as a cooling zone, then a rigid baffle is used as a rule, as described in U.S. Pat. Nos. 3,680,028 and 4,763,716, UK Patent Application Nos. 1,285,319 and 1,562,368, and EP 0589508, Patentschrift 4022389. When cooling is performed by a liquid metal bath, then, besides the rigid baffle, one can use a baffle floating on the surface of the coolant (see German Patent 4,321,640; PCT Application 096/05006, Russian Federation Patent 1,401,715).
The horizontal shields or baffles create a heat barrier between the heating zone and the cooling zone in the apparatuses in order to increase the thermal gradient at the crystal growth front of the cast article (the blade). But when it is necessary to produce blades having large horizontal shoulders, the construction of said shields or baffles should correspond to the maximum size of a blade in a horizontal direction, which decreases the shielding effect and does not ensure the desired crystal structure.
The closest to the present invention is the apparatus disclosed in UK Patent Application No. 1,303,038 being accepted as a prototype. The prototype apparatus consists of a vacuum chamber, inside which a furnace for mold preheating is disposed. Disposed inside the furnace is a ceramic cluster of several blades being positioned on a chill plate. Placed between the cluster central part and a casting cavity is a closed shield which has the shape of a circular cavity. Said shield is produced jointly with a ceramic mold, is rigidly connected with it and assists in uniform heating of all the castings in a cluster along its height and also assists in decreasing the mutual effect of the ceramic cluster on the cooling process.
All the above mentioned apparatuses with shields or baffles, including the prototype apparatus, in the process of lowering the mold into the cooling zone from the heating zone, do not decrease the radial component of the thermal gradient and are not acceptable for casting blades with large horizontal shoulders. As a result structural defects and increased porosity in the transition portion of the blade between the shroud and an airfoil are caused.
The technical task of the present invention is to improve the structure of the casting (i.e., the absence of high and low angle grain boundaries) due to decreasing the radial component of the thermal gradient without changing the temperature axial gradient value in the course of the solidification process. Another aim of the invention is also to improve the yield of single crystal structure in the blade. To achieve this task, the inventive apparatus comprises a vacuum chamber inside which there is positioned a mold preheating furnace with a ceramic mold and a vertical shield disposed therein, and a crystallizer (cooling zone). The vertical shield is made separate from the ceramic mold and is positioned concentrically to the casting""s vertical axis; its lower butt end is open and faces the crystallizer. The shield can be made integral of thin graphitized foil having graphite bushes on the butt ends, or can be made as a row of members having projecting flanges which telescopically insert into each other. The number and the height of these members are defined constructively and are dependent on blade sizes. The shield is fixed on the mold upper portion or on a mold hanger. The shield should be made of a heat insulative material such as, but not limited to, graphitized foil, layers of graphite with felt, and composite materials based on carbon/carbon.